Method to reduce or prevent scaling

ABSTRACT

A method of reducing scale formation in an aqueous solution using an electrolysis apparatus which has, in an electrolysis chamber, at least two electrodes and a bipolar electrode between the two electrodes includes feeding an aqueous solution to the electrolysis chamber, applying either a DC potential to the two electrodes so that one electrode is an anode and the other electrode is a cathode and reversing the polarity of the electrodes at intervals so that the composition of the aqueous solution remains essentially unchanged, or applying an AC potential to the two electrodes, producing, after the aqueous solution has passed through the electrolysis chamber, a treated aqueous solution having a significantly reduced tendency to form scale.

This is a national stage application of PCT/IB97/01243 filed Oct. 8,1997.

BACKGROUND OF THE INVENTION

The invention relates to a method of reducing or preventing scaleformation from aqueous solutions and also to an electrolysis apparatusfor implementing this method.

The formation of scale from aqueous solutions is attributable mainly tothe calcium and magnesium content of natural water and presents ageneral problem both in industrial and commercial operations and in thehousehold since it causes a considerable need for maintenance andshortens the life of the appliances. To avoid these problems, it isusual to add chemicals which prevent scaling or to remove the substanceswhich lead to scale formation, e.g. by means of ion-exchange processes,reverse osmosis, electrolysis and the like.

It is also known that the use of bipolar electrodes which can beconfigured as a fixed bed of conductive and nonconductive particles oras a fluidized bed of conductive particles makes possible higherspace-time yields in electrolysis processes. Such processes aredescribed, for example, in Electrochimica Acta 22, 347-352 (1977) and inElectrochimica Acta 22, 1087-1091 (1977) for preparing hypobromite, forthe epoxidation of styrene, for the synthesis of the dimethyl ester ofsebacic acid and for preparing hypochlorite from seawater. However,bipolar electrodes have not hitherto been used in water treatment.

Furthermore, GB-A-1 409 419 discloses a method of rendering pollutantssuch as chromic acid, cyanide or nitrate in aqueous solutions harmlessby means of electrolysis using a bipolar fixed-bed electrode, in whichthere is added to the electrolyte a compound which reacts, or whosereaction product reacts, with the pollutant to form a compound which isnot harmful. The bipolar electrode comprises nonmetallic, electricallyconductive particles and can preferably also contain nonconductiveparticles.

An electrolysis cell having a bipolar fixed-bed electrode which can beopened at the upper end by means of a flap to allow easy replacement ofthe fixed bed is described in JP-A-04/027 491.

Electrochemical removal of contaminating ions from an aqueous medium isalso described in U.S. Pat. No. 4,123,339, but this method useselectrodes made of iron or an insoluble iron compound and iron ions arereleased at the anode and hydroxide is formed at the cathode under theaction of direct current, which is said to result in reaction with thecontaminating ions to form an insoluble material which can be separatedoff. Uniform consumption of the electrodes can be achieved by reversalof the polarity.

U.S. Pat. No. 3,915,822 describes an electrochemical cell which, in thereaction zone, contains at least one bed of electrically conductiveparticles and has a plurality of electrodes which define adjacentchemical sections in which different voltage gradients can bemaintained. The cell is said to be suitable for metal recovery, foradsorption and desorption of organic compounds, for oxidation ofwastewater, for the synthesis of organic and inorganic compounds and thelike.

DE-C-41 07 708 proposes a method of treating flowing water to preventlimescale by cavitation and an electric AC field, in which cavitationforms zones in which the pressure is significantly below the surroundingpressure in the water to be treated, which is said to result in localoutgassing of the CO₂ dissolved in the water and thus in a disturbanceof the lime-carbon dioxide equilibrium and a decrease in the calciumsolubility, and in which the water which has been treated in this way ispassed between at least two electrodes. The use of electrodes having astructured surface, e.g. knobs, is said to make it possible to achievethe desired treatment effect at low voltages. However, it has been foundin practice that the reduction in limescale which can be achieved bythis method is barely significant.

Furthermore, EP-A-0 171 357 discloses an electrochemical method ofsoftening water in which the alkaline pH in the vicinity of the cathodeeffects precipitation of the Ca and Mg ions in the form of their oxidesand hydroxides which deposit on a porous inert material located betweenthe electrodes. The porous material can be regenerated by reversing thepolarity of the electrodes.

Since the known methods by means of which scale formation from aqueoussolutions can be effectively prevented or reduced involve the additionof chemicals or the complete or substantial removal of the materialsleading to scale formation they are often not very suitable, if at all,for water treatment. In particular, in the treatment of drinking waterit is generally desirable for the natural salt content of the drinkingwater to be changed only slightly or not at all.

SUMMARY OF THE INVENTION

It has now surprisingly been found that scaling can be considerablyreduced or prevented entirely if the aqueous solution is pretreated inan electrolysis apparatus which has a bipolar electrode and in which thedirection of the direct current is periodically reversed. In comparativeexperiments, it was found, for example, that scale formation in adownstream boiler can be largely or completely avoided by means of suchpretreatment. This finding is particularly surprising because the effectalso occurs when the salt content and the pH of the water remainvirtually unchanged. Furthermore, it was surprisingly found that theeffect also occurs when an AC voltage is applied to the electrodes.There is as yet no explanation for this effect. It is possible that aslight shift in the lime-carbon dioxide equilibrium slows the kineticsof calcite precipitation so that scale formation is significantlyreduced or prevented at the customary residence times in pipes and waterheaters.

The present invention accordingly provides a method of reducing orpreventing scale formation from aqueous solutions, in which the aqueoussolution which tends to form scale is fed to an electrolysis chamberwhich has at least two electrodes and in addition a bipolar electrodebetween the electrodes and, after passing through the electrolysischamber, a treated aqueous solution having a significantly reducedtendency to form scale is obtained, with the method being characterizedin that a DC potential is applied to the electrodes so that at least oneelectrode acts as anode and at least one electrode acts as cathode andthe polarity of the electrodes is reversed at intervals of time in sucha way that the composition of the water fed in remains essentiallyunchanged, or in that an AC potential is applied to the electrodes.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of an electrolysis apparatus accordingto the invention.

DETAILED DESCRIPTION

For the purposes or the present invention, the expressions “electrode”and “electrolysis chamber” are, because of the substantial analogy ofthe process parameters and the equipment which can be employed, usedboth for DC operation and for AC operation and encompass electrodes andelectrolysis chambers as are known to those skilled in the art fromelectrolysis processes or are described below. For the purposes of thepresent invention, the expressions “AC potential” and “alternatingcurrent” encompass, in particular, AC potentials and alternatingcurrents of customary frequencies, typically about 50 Hz. Theessentially unchanged composition of the water means, for the purposesof the present invention, that the salt content and in particular thecontent of salts which tend to form scale is not significantly differentin the treated aqueous solution from that in the aqueous feed solutionwhich tends to form scale. Since the method of the invention makes itpossible to essentially avoid removal of the dissolved salts, the totalhardness of the treated solution does not differ significantly from thatof the feed solution.

The method of the invention provides an essentially maintenance-free wayof reducing or completely preventing scale formation from aqueoussolutions which tend to form scale without the salt content or the pH ofthe solution being changed significantly or chemical water softenershaving to be added and without salts such as calcite or excessivelyhardened water being obtained as waste. It is in principle suitable forthe treatment of any aqueous solutions which tend to form scale. Aparticularly preferred application area is the treatment of drinkingwater (i.e. the treatment of water whose hardness is essentiallyattributable to the presence of calcium and magnesium ions) where itmakes it possible, despite substantial or complete prevention ofscaling, for the natural water quality to be essentially maintained. Afurther preferred application area is the pretreatment of water forwashing machines, dishwashers and other appliances in which a watersoftener has hitherto usually had to be added.

A preferred aspect of the method of the invention is therefore thetreatment of water having a natural content of carbonates,hydrogencarbonates and sulphates of calcium and magnesium.

The method of the invention makes it possible to prevent salt depositssuch as calcite being obtained as waste, either as such or, as a resultof reversing the polarity of the electrodes to remove deposits, in theform of excessively hardened water, i.e. it is characterized in that thetotal amount of the aqueous solution fed to the electrolysis chamber hasa significantly reduced tendency to form scale after passing through theelectrolysis chamber and essentially no calcite or excessively hardwater is obtained as waste. Since removal of the dissolved salts canessentially be avoided by means of the new method, the aqueous solutiontreated according to the invention has a total hardness which ispreferably not more than about 1° dH (German degree of hardness), inparticular not more than about 0.5° dH, below that of the aqueoussolution fed in. Since the pH of the aqueous solution is also notchanged significantly according to the method of the invention, thetreated solution has a pH which differs from that of the feed solutionby not more than about 0.05.

The abovementioned reversal of the polarity of the electrodes in thecase of direct current operation effects a change in the field directionand the current direction in the electrolysis chamber. It canadvantageously be brought about by reversing the polarity of the DCpotential applied by means of customary regulating and control deviceswhich are well known to those skilled in the art.

The reversal of polarity can be carried out periodically at constanttime intervals, e.g. every 2 seconds, or else at time intervals havingdifferent lengths, for example alternately after relatively short (e.g.30 seconds) and relatively long (e.g. 45 seconds) intervals. The lengthof the time intervals is not critical; in general, however, intervals ofnot more than about 60 seconds, preferably from about 1 to 60 seconds,have been found to be useful.

In the simplest and preferred embodiment, the direct electric currentcan be fed in and taken off via two electrodes which are alternatelyconnected as cathode and anode. If desired, it is also possible to usemore than two electrodes, for example three electrodes, of which two areconnected as anode and one as cathode, or four electrodes of which twoare connected as anode and two function as cathode. In an analogous way,two or more electrodes can also be used for AC operation.

The optimum value of the DC or AC voltage is dependent, inter alia, onthe electrode spacing, but is usually in the range from about 5 to 20 Vper cm between the electrodes. The flow rate of the aqueous solution andthe electric current are dependent on the dimensions of the apparatus,the salt content, the voltage and the like, but are not critical. Ingeneral, however, the ratio of the electric current flowing through theelectrolysis chamber to the flow rate of the aqueous solution fed in ismade not more than about 2 A·h/m³, with preference being given to arange from about 0.5 to 1.5A·h/m³, in particular from about 1.0 to 1.3A·h/m³. In the method of the invention, it is also generally preferredfor the current to be kept constant. For example, a current of not morethan about 4 A, preferably not more than about 2 A, has been found to beuseful for small apparatus having an electrode spacing of about 2 cm.Typically, the current is about 1-2 A, the voltage is about 20-40 V andthe flow rate is from about 0.05 to 3 m³/h.

When the apparatus is put into operation for the first time, aconsiderable reduction in the tendency to form scale is generallyestablished only gradually, which might be attributable to theachievable effect only coming to bear fully when a certain minimumamount of calcite has been deposited on the electrodes and the bipolarelectrode. However, it has been found that the optimum effect isgenerally achieved after only about 3-4 m³ of aqueous solution haveflowed through if, when the electrolysis chamber is put into operationfor the first time or after the electrolysis chamber has been cleaned(or the bipolar electrode has been replaced), an aqueous solution whichtends to form scale is fed to the electrolysis chamber, a DC voltage isapplied to the electrodes and the polarity of the electrodes is reversedalternately at relatively short and relatively long time intervals untilthe treated aqueous solution displays a significantly reduced tendencyto form scale.

Suitable apparatus for implementation of the method of the invention areelectrolysis units comprising a water inlet system, a water outletsystem and an electrolysis chamber having at least two electrodes ofwhich, in the case of application of a DC potential, at least one actsas cathode and at least one acts as anode, and a bipolar electrodelocated between the electrodes, with the apparatus also beingcharacterized in that the electrodes are either connected to an ACsource or are connected via a regulating unit to a DC source in such away that the polarity of the electrodes can be reversed at intervals oftime. Apparatus operated by means of alternating current are new and arelikewise subject matter of the present invention. The inventiontherefore likewise provides an electrolysis apparatus comprising a waterinlet system, a water outlet system and an electrolysis chamber having aleast two electrodes and a bipolar electrode located between theelectrodes, with the apparatus additionally being characterized in thatthe electrodes are connected to an AC source. The following remarksapply, unless expressly indicated otherwise, both to an electrolysisapparatus operated using alternating current and to an apparatusoperated using direct current.

The electrolysis chamber is preferably separated from the water inletsystem and from the water outlet system by an envelope provided withopenings. The size and shape of the openings are preferably selectedsuch that the flow of the aqueous solution is hindered as little aspossible but, on the other hand, when using a bipolar particleelectrode, virtually no particles can escape. The envelope preferablycomprises plastic and the openings preferably have the shape of smallround holes or small slits. The water pressure is not critical for themethod of the invention. However, the electrolysis chamber and the waterinlet and water outlet systems are preferably of such a constructionthat the pressure drop is very small.

Suitable electrode materials are essentially all materials which arecustomarily used as long as their use in water treatment is acceptable.In general, preference is given to graphite. However, it is likewisepossible to use other materials such as noble metals or titanium steelcoated with noble metals or with mixed oxides. The electrode spacing isnot critical and can be, for example, about 2 cm.

The DC or AC potential applied to the electrodes can, as a matter ofchoice, act perpendicular or parallel to the flow direction of theaqueous solution.

Suitable bipolar electrodes are likewise known to those skilled in theart. According to a preferred embodiment, the bipolar electrode locatedin the space between the electrodes can be, for example, a fixed-bedelectrode, where the fixed bed can, in particular, comprise electricallyconductive particles and nonconductive particles. Suitable electricallyconductive particles are, for example, graphite, activated carbon,synthetic carbons and noble metals or other metals which release noions; activated carbon and especially graphite have been found to beparticularly suitable. Suitable nonconductive particles are essentiallyany inert and water-insoluble nonconductive materials, in particularsilica, glass and plastics. The particle size of the conductive andnonconductive particles is not critical; however, materials having amean particle size of from about 0.5 to 2 mm are generally preferred. Toavoid the risk of short circuits, the volume ratio of the conductiveparticles to the nonconductive particles in the fixed bed shouldpreferably be not more than about 1:1; particular preference isgenerally given to a ratio of about 1:2. When using a fixed bed, theelectrodes are preferably introduced directly into the fixed bed and theaqueous solution preferably flows from the top downwards or horizontallythrough the fixed bed.

In place of a particle bed, it is also possible to use, for example,graphite rods which are provided with rings of insulating material suchas nylon to avoid short circuits and are arranged as a stack. Such abipolar electrode is described, for example, in Electrochimica Acta 22,347-352 (1977).

In a further preferred embodiment, the bipolar electrode can beconfigured as a fluidized bed of electrically conductive particles; inthis embodiment, the aqueous solution is preferably passed through thefluidized bed from the bottom upwards. In this embodiment, the flow rateis preferably made high enough for the flow alone to produce sufficientfluidization. Since short circuits are avoided as a result of thefluidization, the use of nonconductive particles is in principlesuperfluous. However, if desired, the fluidized bed can also containnonconductive particles in addition to the electrically conductiveparticles. In this case, the materials mentioned above in connectionwith the fixed bed are preferred. Particular preference is given to theuse of a fluidized bed which consists of only graphite particles.

Further preferred bipolar electrodes are porous sheets of electricallyconductive material which are arranged parallel to the electrodes. Tokeep the sheets separate from one another and from the electrodes,spacers made of a nonconductive material such as plastic, glass and thelike are preferably used.

Unlike the case of three-dimensional electrodes, the electric current inthe case of bipolar electrodes is always forced to travel both throughthe electrically conductive material of the bipolar electrode andthrough the liquid medium. In the interstices between inert material andconductive material and in the pores of the conductive material, veryhigh electric field intensities of several thousand V/m arise. It isknown from studies by Onsager (J. Chem. Phys. 2, 599-615 (1934)) thatthese high field intensities can lead to changes in the dissociationconstants. It is possible that the high field intensities cause anincreased alkalinity which could result in formation of crystal nuclei.

Basically, the method of the invention results, when using directcurrent, in the same electrochemical reactions at the anode and theanodic surfaces of the bipolar electrode and at the cathode and thecathodic surfaces of the bipolar electrode as is the case in normalelectrolysis, namely the formation of oxygen from water and of carbondioxide from bicarbonate at the anode and the formation of hydrogen fromwater and of carbonate from bicarbonate at the cathode. The oxidationoccurring at the anode and the anodic surfaces of the bipolar electrodemight then also be a reason for the simultaneous denucleating effect ofthe method of the invention. Furthermore, the formation of carbonatecauses the precipitation of calcite and the magnesium ions present inthe water are precipitated in the interstices of the bipolar electrodeas magnesium hydroxide.

However, in the method of the invention, reversing the polarity of theelectrodes brings precipitates formed back into solution, thuseffectively preventing blocking of the fixed-bed electrode and alsoavoiding a significant change in the salt content of the aqueoussolution. However, if desired, the method can also be implemented insuch a way that partial precipitation of the salts takes place at thesame time. If, as mentioned above, the polarity is reversed atalternately relatively short and relatively long time intervals, theprecipitates no longer go completely into solution and some of themremain in the electrolysis apparatus. Since the proportion ofirreversible precipitation increases with increasing difference in theintervals, the desired salt content can easily be set in this way.

The interaction of the above-described mechanisms can therefore enable,according to the invention, scale formation or calcite deposition to besignificantly reduced or even prevented entirely and can at the sametime enable the composition of the water to be left essentiallyunchanged or, if desired, its salt content to be reduced in a targetedway.

A preferred embodiment of the electrolysis apparatus of the inventionfor operation using direct current is shown in longitudinal section inFIG. 1; for the sake of clarity, the housing 1, the connection head 2and the interchangeable cartridge 3 are depicted individually.

FIG. 1 shows an interchangeable cartridge 3 having a chamber 4 for theuntreated aqueous solution, a chamber 5 for the treated aqueous solutionand an electrode chamber in which two graphite electrodes 6 and abipolar fixed-bed electrode 7 of electrically conductive andnonconductive particles are arranged and which is separated from thechambers for the untreated and treated aqueous solutions by means of, ineach case, an envelope 8, 9 having openings in the form of small holesor slits. The electrodes 6 are connected via power connections 10 to aregulating unit (not shown) which allows the polarity of the electrodesto be reversed at intervals of time and which is in turn connected to adirect current source. The cartridge 3 is, as indicated by the arrow andthe axis of the apparatus drawn in as a broken line, pushed from thebottom into the housing 1 and fixed in place by means of a screw closure11. The housing 1 consists essentially of an outer tube 12 and has atits other end a conductive metal plate 13 for screening off the electricfield and a locking ring 14 which can be moved upwards and serves tofasten the connection head 2 having an inlet opening 15 for theuntreated aqueous solution and an outlet opening 16 for the treatedaqueous solution to the housing 1. The cartridge 3 preferably has anouter envelope 17 which closes off the outside of the chambers 4, 5 forthe aqueous solution. If such an envelope 17 is not present, the outertube 12 assumes this function. The outer tube 12 and the outer envelope17 of the cartridge 3 are cylindrical in shape and the envelopes 8, 9provided with openings preferably also together have at leastapproximately the shape of a cylinder, with the chambers 4, 5 for theuntreated and the treated solutions being separated from one another bydividing walls which are above or below the plane of the drawing and arelocated between the envelopes 8, 9 and the envelope 17 or the outer tube12.

When an aqueous solution is fed into the apparatus shown in FIG. 1 viathe inlet opening 15, the solution flows from chamber 4 through theopenings in the envelope 8 into the electrode chamber, flows essentiallyparallel to the electric field through the fixed bed and, after passingthrough the openings in the envelope 9, flows out through the chamber 5and the outlet opening 16.

An apparatus for operation using alternating current can essentially berealized in an analogous way, but preference is generally given tosmaller electrode areas and a larger electrode spacing for AC operationthan for DC operation.

The method of the invention and its effect are illustrated by means ofthe following examples.

EXAMPLE 1

Drinking water from the municipality of Mondsee (Austria) was dividedinto two equal substreams. One substream was passed through areversible-polarity electrolysis apparatus and subsequently through ahot water boiler. The other substream was passed without pretreatmentdirectly through a hot water boiler of the same make. Both boilers werefitted with heating coils having the same heating power and identicalregulators and measuring instruments were used, with PID regulators(Proportional-Integral-Differential regulators) being used forregulating the temperature.

As electrolysis apparatus, use was made of an apparatus as shown in FIG.1 having two graphite electrodes, an electrode spacing of 2 cm and abipolar fixed-bed electrode of graphite particles and silica in a volumeratio of 1:2. The experiments were carried out at a DC voltage of 40 Vand the polarity was reversed alternately at intervals of 30 and 45seconds. The throughput of drinking water was 500 l/h. The boilertemperature was regulated to 70° C. in both boilers.

After about 400 m³ of water had flowed through each boiler, both boilerswere acidified with nitric acid and the amount of calcite deposited inthe boilers was determined.

The drinking water from the municipality of Mondsee which was used had atotal hardness of 16.5° dH, a carbonate hardness of 15.5° dH, anelectric conductivity of 530 μS/cm (25° C.), a pH of 7.5 (20° C.) and anacid capacity up to pH 4.3 of 5.5 mmol/l. The following values arecalculated from these figures in accordance with DIN 38404-10:

Calculation temperature: 25° C. 70° C. Buffering intensity: 0.86 mmol/l1.09 mmol/l Saturation index: 0.46 0.83 Calcite saturation pH: 7.13 6.79Calcite deposition capacity: 34.5 mg/l 74.87 mg/l

As the calculations show, the calcite deposition capacity is about 75mg/l.

The calculation of the effectiveness was carried out as indicated in theDVGW test method “Prüfverfahren zur Beurteilung der Wirksamkeit vonWasserbehandlungsanlagen zur Verminderung von Steinbildung”. On thebasis of the abovementioned comparative experiments, an effectivenessfactor of 98.5% was obtained for the water in Mondsee.

EXAMPLE 2

The experiment was carried out using drinking water from themunicipality of Schriesheim (Germany). Here, a stream of 1 m³/h waspassed continuously through a reversible-polarity electrolysis apparatusas described in Example 1 and a substream of 20 l/h of the treated waterwas fed to a hot water boiler. The DC voltage applied to theelectrolysis apparatus was 35 V and the polarity was reversedalternately at intervals of 30 and 45 sec. A stream of likewise 20 l/hwas passed through a blank section, i.e. without pretreatment of thewater, and directly through a second hot water boiler. Both sections,i.e. the section with pretreatment and the blank section, were providedwith hot water boilers of the same make and heating coils having thesame electric power. In both sections, the boiler temperature wasregulated to a temperature of 80° C. using identical regulators. Thetotal duration of the experiment was 21 days.

The drinking water was analysed before and after passing through theelectrolysis apparatus, giving the following values:

before after treatment treatment Calcium 135 mg/l 134 mg/l Magnesium22.3 mg/l 22.7 mg/l Sodium 21.3 mg/l 21.4 mg/l Chloride 69 mg/l 69 mg/lSulphate 79.2 mg/l 77.7 mg/l Nitrate 36.6 mg/l 36.6 mg/l pH (20° C.)7.38 7.37 Conductivity (20° C.) 109.2 mS/m 109.4 mS/m Acid capacity upto pH 4.3 5.36 mol/m³ 5.5 mol/m³

The analytical results show that the pretreatment in the electrolysisapparatus causes no appreciable chemical change in the drinking water.The evaluation of the amounts of lime in the two boilers (using themethod described in Example 1) indicated that scale formation wasprevented to an effectiveness factor of 92.2% by the pretreatment, i.e.scaling could be largely avoided while maintaining the water quality.

What is claimed is:
 1. A method of reducing scale formation in anaqueous solution using an electrolysis apparatus which has, in anelectrolysis chamber, two electrodes and a bipolar electrode between thetwo electrodes, the method including feeding an aqueous solution whichtends to form scale into the electrolysis chamber, and applying either aDC potential to the two electrodes so that one of the two electrodes isan anode and one of the two electrodes is a cathode and reversing thepolarity of the electrodes at intervals, or applying an AC potential tothe two electrodes, thereby producing, after the aqueous solution haspassed through the electrolysis chamber, a treated aqueous solutionhaving a significantly reduced tendency to form scale and a compositionessentially unchanged from that of the aqueous solution.
 2. The methodaccording to claim 1 including feeding water having a natural content ofcarbonates, hydrocarbonates, and sulphates of calcium and magnesium intothe electrolysis chamber.
 3. The method according to claim 1, whereinthe aqueous solution fed into the electrolysis chamber produces, afterpassing through the electrolysis chamber, essentially no calcite and noexcessively hard water as waste.
 4. The method according to claim 1,wherein the total hardness of the treated aqueous solution is notreduced by more than 1° dH, compared to the total hardness of theaqueous solution fed into the electrolysis chamber.
 5. The methodaccording to claim 1, wherein the pH of the treated aqueous solutiondiffers by not more than 0.05 from that of the aqueous solution fed intothe electrolysis chamber.
 6. The method according to claim 1, includingapplying the DC potential to the two electrodes and reversing thepolarity of the two electrodes at uniform intervals of time.
 7. Themethod according to claim 1, including applying the DC potential to thetwo electrodes and reversing the polarity of the two electrodes atalternately relatively short and relatively long time intervals.
 8. Themethod according to claim 1, including applying the DC potential to thetwo electrodes and reversing the polarity of the two electrodes at timeintervals of from 1 to 60 seconds.
 9. The method according to claim 1,wherein the electric current flowing through the electrolysis chamberdivided by the flow rate of the aqueous solution fed into theelectrolysis chamber is not more than 2 A·h/m³.
 10. The method accordingto claim 9, wherein the electric current flowing through theelectrolysis chamber divided by the flow rate of the aqueous solutionfed into the electrolysis chamber is from 1.0 to 1.3 A·h/m³.
 11. Themethod according to claim 1, wherein the electric current flowingthrough the electrolysis chamber is not more than 4 A.
 12. The methodaccording to claim 1, including applying a DC or AC field of 5-20 V percm between the two electrodes.
 13. The method according to claim 1,wherein the two electrodes are selected from the group consisting ofgraphite, noble metals, and titanium steel coated with a coatingselected from the group consisting of noble metals and mixed oxides. 14.The method according to claim 1, wherein the two electrodes areseparated by a distance of about 2 cm from each other.
 15. The methodaccording to claim 1, wherein the bipolar electrode is a bipolarfixed-bed electrode.
 16. The method according to claim 15, wherein thebipolar fixed-bed electrode is a fixed bed of electrically conductiveparticles and nonconductive particles.
 17. The method according to claim16, wherein the electrically conductive particles are selected from thegroup consisting of graphite particles and activated carbon particles,and the nonconductive particles are selected from the group consistingof silica particles, glass particles, and plastic particles.
 18. Themethod according to claim 16, wherein the electrically conductiveparticles and the nonconductive particles are present in the fixed bedin a volume ratio of not more than 1:1.
 19. The method according toclaim 1, wherein the bipolar electrode comprises porous sheets ofelectrically conductive material arranged parallel to the twoelectrodes.
 20. The method according to claim 19, wherein the sheets areseparated from each other and from the two electrodes by nonconductivespacers.
 21. The method according to claim 1, wherein the bipolarelectrode is a fluidized bed of electrically conductive particles andthe aqueous solution flows upwards through the fluidized bed.
 22. Themethod according to claim 1, wherein the electrolysis apparatus has awater inlet and a water outlet separated from the electrolysis chamberby an envelope including openings.
 23. The method according to claim 1,comprising pretreating the electrolysis apparatus, including feeding anaqueous solution which tends to form scale into the electrolysischamber, applying a DC potential to the two electrodes, and reversingthe polarity of the two electrodes alternately at relatively short andrelatively long time intervals until the treated aqueous solution has asignificantly lower tendency to form scale.